I know the search function is down right now, but there are a bunch of threads from last year on this topic, containing lots of info. Check em out, especially if you want to get an idea of when people got in.
Unfortunately though, no one really knows much about Duke's waitlist, aside from those working in admissions themselves. I was lucky enough to get in off the waitlist last year, and from talking to my classmates I figure that there are somewhere between 7-10 of us, but it varies from class to class. Some got in right after May 15th, and some got in only a few days before classes started in early August!
I've also heard that the list is AT LEAST 100 applicants long, and that it's solidly ranked, meaning that sending letters, etc. won't necessarily help you. But I sent letters and stuff anyway, becuase I figured it couldn't hurt, and I really really loved Duke, so I had to give it a try. Honestly, I don't know if it helped. And I don't know if this information is accurate either...it sucks that they won't divulge any info.
They're so tight-lipped about it that they can't even tell you if you're in the top or bottom half of the list! I found this really frustrating and unfair, but c'est la vie.
Sorry I couldn't be more helpful. Good luck with the waitlist! Hopefully you won't have to wait until July like I did!